A high-strength steel sheet is disclosed having a specified chemical composition and a steel microstructure composed of, on an area fraction basis, ferrite: 1% to 40%, fresh martensite: 1% to 20%, bainite and tempered martensite in total: 35% to 90%, and retained austenite: 6% or more, wherein a value obtained by dividing an average Mn content (% by mass) of the retained austenite by an average Mn content (% by mass) of the ferrite is 1.1 or more, and a value obtained by dividing an average C content (% by mass) of retained austenite with an aspect ratio of 2.0 or more by an average C content (% by mass) of the ferrite is 3.0 or more, and a value obtained by dividing a C content of all retained austenite by a C content of a T.sub.0 composition is 1.0 or more.

A high-strength steel sheet is disclosed having a specified chemical composition and a steel microstructure composed of, on an area fraction basis, ferrite: 1% to 40%, fresh martensite: less than 1.0%, bainite and tempered martensite in total: 40% to 90%, and retained austenite: 6% or more, wherein a value obtained by dividing an average Mn content (% by mass) of the retained austenite by an average Mn content (% by mass) of the ferrite is 1.1 or more, and a value obtained by dividing an average C content (% by mass) of retained austenite with an aspect ratio of 2.0 or more by an average C content (% by mass) of the ferrite is 3.0 or more, and a diffusible hydrogen content of steel is 0.3 ppm by mass or less.

A high-strength steel sheet having a TS of 780 MPa or more, excellent stretch flangeability, and excellent in-plane anisotropy of TS is provided. A high-strength steel sheet comprises: a predetermined chemical composition; a steel microstructure including, in area fraction, ferrite: 20% or more and 50% or less, lower bainite: 5% or more and 40% or less, martensite: 1% or more and 20% or less, and tempered martensite: 20% or less, and including, in volume fraction, retained austenite: 5% or more, the retained austenite having an average grain size of 2 m or less; and a texture having an inverse intensity ratio of -fiber to -fiber of 3.0 or less.

A high-strength steel sheet having a TS of 780 MPa or more, not only excellent ductility but also excellent stretch flangeability, and excellent in-plane anisotropy of TS is provided. A high-strength steel sheet comprises: a predetermined chemical composition; a steel microstructure including, in area fraction, ferrite: 20% or more and 50% or less, upper bainite: 5% or more and 45% or less, and martensite: 1% or more and 20% or less, and including, in volume fraction, retained austenite: 5% or more, the retained austenite having an average grain size of 2 m or less; and a texture having an inverse intensity ratio of -fiber to -fiber of 3.0 or less.

Provided are a hot dip galvanized steel sheet comprising a base steel sheet wherein the base steel sheet has a predetermined composition and contains ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, remaining structures consist of bainite, when defining a region having a hardness of 90% or less of the hardness at a position of ? thickness to the base steel sheet side from an interface of the base steel sheet and a hot dip galvanized layer as a soft layer, there is a soft layer having a thickness of 10 ?m or more at the base steel sheet side from the interface, the soft layer contains tempered martensite, and an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/?m or less, and a method for producing the same.

A surface-treated steel sheet is provided that has excellent strength even when the surface-treated steel sheet is provided with a NiCoFe alloy layer. A surface-treated steel sheet of the present disclosure includes a steel sheet, and a NiCoFe alloy layer on a steel sheet surface. The steel sheet has a microstructure containing: ferrite, and one or more kinds selected from the group consisting of cementite, pearlite, and bainite having an area fraction of 1.2 to 5.0% in total. The NiCoFe alloy layer contains Ni, Co and Fe.

A steel part having a homogeneous multiphase microstructure in each region of the part, the microstructure containing ferrite, wherein the steel part is obtained by a process involving: cutting a blank from a strip of steel, having a specified composition; optionally, the blank undergoes prior cold deformation; the blank is heated to reach a soak temperature T.sub.s above Ac1 but below Ac3 and held at this soak temperature T.sub.s for a soak time t.sub.s adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% by area; the heated blank is transferred into a forming tool to hot-form the part; and the part is cooled within the tool at a cooling rate V such that the microstructure of the steel, after cooling the part, is a multiphase microstructure containing ferrite and being homogeneous in each region of the part.

Provided is a hot-formed member according to the present invention, having a predetermined chemical composition and having a metallographic microstructure containing austenite at an area ratio of 10 area % to 40 area % and in which total number density of particles of the austenite and martensite is equal to or greater than 1.0 number/?m.sup.2, in which tensile strength is from 900 MPa to 1300 MPa.

A method of producing a steel plate member including: quenching a steel plate by heating a steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed, and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate; reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and cooling the steel plate at a cooling rate lower than a lower critical cooling rate.

One aspect of the present invention is a high strength steel sheet having a specific component composition, wherein a metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained austenite; when the metal structure is observed with a scanning electron microscope, the metal structure satisfies polygonal ferrite: 10 to 50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area % with respect to the metal structure overall; and when the metal structure is measured by X-ray diffractometry, the metal structure satisfies retained austenite: 5.0 volume % or more, retained austenite with a carbon concentration of 1.0 mass % or less: 3.5 volume % or more, and retained austenite with a carbon concentration of 0.8 mass % or less: 2.4 volume % or less with respect to the metal structure overall.